Didactics - 13 : Evolution of the Human Brain
A : Introduction
For the next few weeks I want to talk about the psychology of the learning process, and in particular about the psychology of the adolescent. But before I do that, I would like to stop for a moment and consider how the human mind emerged through the process of biological evolution #1. . The story that I am going to tell you is largely speculative, and should be taken as illustrative rather than factual. I would like to believe that it will lead you to reflect about the place of language in human society - about its functions and its functioning.
B : The Human Animal
To understand the human being, we need to begin with the fact that he is an animal. We have evolved. Evolution works through selection. If an animal is born with a characteristic that makes it more likely to survive than its brothers, sisters and cousins, it will have more offspring. The gene that gave it that characteristic will spread through the population, and those who do not possess the gene will gradually die out. But evolution can only build upon what exists - we carry traces in our bodies of the conditions that shaped our ancestors. Sometimes, these traces appear to have no function at all for us now - the appendix, for example, does not seem to do anything that we need, but is very useful for herbivorous mammals such as the rabbit. Sometimes the function has changed; an ape's feet provide him with a second pair of hands, which he uses for grasping objects and for climbing trees. Man uses his feet for standing on and walking. This shows us that evolution is an opportunistic process; often accidental characteristics can be transformed into central ones - human beings are among the animals that are designated neotenic- that is to say that we mature slowly, and in fact retain many juvenile characteristics well into adult life. One of these is a large head - and hence, a large brain - we have large brains not because they fulfilled any particularly useful function, but because neoteny fulfilled a useful function, and the large brain came with it #2.
We are mammals - that is to say that we are descended from small, furry creatures that were mainly nocturnal - thus avoiding being stomped on by dinosaurs. For reasons that we will see, these creatures became more intelligent - or at least developed larger brains - largely because of their nocturnal habits. We are monkeys - that is to say that our ancestors were arboreal creatures, swinging from tree to tree, living mainly off fruit. It is thanks to the monkeys' way of life that our forebears developed 3-D vision, were able to see in colour, and developed that marvel of genetic engineering, which sets us and our simian cousins apart from all almost other animals - the hand with an opposable thumb.
We then found ourselves ejected from the forests, living out in the open, in competition with the large specialist predators such as the leopard and the lion. We ate what we could find - the powerful jaws and large teeth of our ancestors were developed for grinding grain and roots, rather than tearing flesh. But humans became omnivorous - meat began to make up a considerable part of their diet - today, hunter/gatherer peoples obtain about one third of their nutritional resources from animal protein. This also had an effect on the way that we are.
C : Pathways to the human brain
To understand the mind, we may as well start with the brain - although, as we shall see, the human mind cannot be simply equated with the brain. If we look at the lower vertebrates, such as fish and reptiles, we may see that they have a relatively small brain - on average, about one quarter the size of the mammalian one. Reptiles' main information input system is visual, and a large part of the processing for visual information actually takes place in the eye itself, which is well equipped with nerve cells. As reptilian life developed, two broad branches could be identified; one of these was highly successful, and gave rise eventually to such creatures as the dinosaurs. The other branch was rather less successful, and evolved into creatures that mainly subsisted at night - these were the animals that evolved into mammals.
The original mammals were small, nocturnal creatures. Obviously, vision could no longer play the role that it does for reptiles, and so they developed greater acuity of hearing and of smell. There was no place for the extra nerve cells needed in the ears, or in the nose, and so it was the brain itself that grew. Moreover, now that the animals were functioning on three input systems - sight, hearing and olfaction - they needed to be able to integrate them, and this called for a more complex nervous system - a bigger brain.
Eventually, the dinosaurs died out, and mammals were able to move back into the daytime world again. This meant that they needed to develop their visual systems once again - however, there are no backward roads in evolution - they did not go back to the reptilian visual system, but developed larger brains, and more highly integrated nervous systems.
Amongst the mammals, certain developed an arboreal existence, living mainly off fruit. This environment favoured three developments :
- - three-dimensional vision - if you cannot judge distances, life jumping from branch to branch can lead to nasty surprises
- - colour vision - the ability to find fruit among all the greenery was important
- - the opposable thumb - this helps in climbing trees - and also allows the animal to develop the use of tools - chimpanzees, for example, use sticks and rocks as tools. It also means that the animal can pick things up and examine them. Get someone to tie your finhers together and try being a dog for an hour.
All of these developments favoured a lager brain, for they needed higher processing skills. Most primates also live in groups - and, as Harry Jerison says, they are noisy animals #3. . They signal to each other through the noises that they make - vervet monkeys, for example, have a large repertoire of specific sounds that have regular meanings - such as 'look out, there's a snake', or 'hey, the food is good over here'. Group life puts a premium on socials skills, such as facial recognition, understanding the emotions of others and so on, all of which demand intelligence, and which may be one of the reasons why primates have brains which are relatively large even for mammals.
The next stage in evolution appears to have been the expulsion of certain primates from the forest. These monkeys took to the veldt #4. , where they were faced with the problem of finding food in large spaces. This may be one of the reasons why hominids adopted the upright position - it enables the animal to see over greater distances. This reliance on the visual faculty was the more important as primates had lost much of their sense of smell while up in the trees.
Jerison suggests, in a speculative piece, that this is where language came in : whereas other wide-ranging animals, such as wolves or hyenas, mark their territory and map their world through smell, hominids cannot do this. He suggests that language developed in order to fulfil this function - that is, that it was the primary evolutionary function of language to enable humans to construct the reality within which they lived. "We need language," he says, "more to tell stories than to direct actions" - it is not communication that is the most important job we do with language, but the construction of mental images. This pressure towards the growth of language would have lead to a further development of the brain, which explains why the human brain is the largest of all the land mammals. We should be careful to take this with a pinch of salt - many paleoanthropologists now believe that the hominid brain was already relatively large before the development of language, and indeed it has even been advanced that language was a kind of accidental result of the growth of the brain, which itself grew for other reasons.
Modelling the brain
Relative to body weight, then, the human brain is the largest possessed by a land animal - the dolphin has a brain which is of similar proportions, or perhaps slightly larger. How does it work? The big question here has been to know whether it is an undifferentiated processing machine, like a computer, or whether different functions are carried out by different organs within the brain. The answer that most scientists would give at the moment is that the brain is a mixture of the two. There is a high degree of specialisation within the brain - different bits do appear to do different jobs - but at the same time, any one task undertaken by a human being will call on more than one area of the brain. Moreover, it now appears that the brain can be pictured in terms of neural networks, which cover wide areas of the physical structure, and which are called into action selectively.
If we look at the human brain from above, we can see it as being clearly divided down the middle into two hemispheres - the right hemisphere and the left hemisphere. The right hemisphere deals with information coming from the left hand side of our sensory mechanisms - the left hand side of the visual field, the left hand, etc. - while the left hand hemisphere deals with information coming from the right hand side. People who have suffered brain damage to the right hand side of the brain, for example, find it difficult to identify objects that are placed in the left hand side of their field of vision.
The two halves of our brain, however, do not both carry out the same functions. To begin with, it often seems that one half of the brain - the left half for most right-handed people - is dominant over the other - that is part of the reason why one hand is dominant over the other. Certainly it appears, that for most right-handed people, the left-hand side of the brain does most of the work in providing that function which we think of as being most fully human - language.
This has been known since the end of the nineteenth century, for in 1861, Paul Broca, Professor of surgical pathology at the Faculté de Medecine, Paris, founder of the Ecole des Hautes Etudes (1858), Paris, and of the Societe d'Anthropologie de Paris (1859), exhibited the brain of one of his patients, known as 'Tan' who had died the day before. Tan had suffered from aphasia - that is to say from a partial or total loss of the powers of speech. When his brain was examined, it was discovered to have been damaged in the posterior part of the left frontal lobe - look at the illustration, and you will find this region labelled Broca's area. People who have lesions in this area of the brain are able to understand what is said to them, and may read, but they are unable to produce fluent sentences. It is as if the grammatical function had been suppressed.
In 1874, Carl Wernicke located the area in the brain that governed another speech function. A lesion in Wernicke's area (the posterior, upper part of the temporal lobe on the dominant side) causes a receptive aphasia. The individual may verbalise extensively, talking in gibberish with occasional mispronounced words. The person is also unable to understand spoken or written language : often, he does not realise what is happening to him, and so does not understand that he is ill. Some people with this kind of aphasia become psychotic, and this could be due to their communication difficulties.
Both the production and the comprehension of grammatical speech appear to be located in the left hemisphere of the brain, then. For a long time, scientists referred to the right hemisphere as the 'silent hemisphere', and it was thought to do very little. However, it is becoming evident that the right hemisphere does have a role to play. It appears to handle spatial intelligence, for example, and houses our musical abilities - which is why aphasics can still sing songs - but it is also involved in the production of speech - people who cannot access the right hemisphere of their brains produce flat, boring speech - the right hemisphere brings colour and emotion to our language. It also appears to be responsible for our sense of humour.
This lateralizationof the brain - where the two hemispheres do different jobs - is peculiar to the more developed mammals, and in particular the apes. It is most developed in human beings. When we are born, it is not fully in place. If a child under the age of eight suffers brain damage to one side, it may be able to replace the lost function, using brain areas on the other side. Scientists refer to this as 'plasticity'. However, it does appear that this plasticity is not complete - children who have suffered early lesions to their language producing areas do use language differently from children who have developed normally, although these differences are not usually noticeable in ordinary conversation.
The brain goes through considerable changes - mostly in the first months of life, but some continue up to the age of about eight years old. It would appear, for example, that the two halves are not fully wired together until about this age, which raises an interesting philosophical problem as to what constitutes 'personality', and when a person can be said to possess a mind. By the way, these changes also involve the deaths of a large number of cells - as specific routes are set up and used within the brain, cells that do not have a function die off.
The different functions appear to set in at different periods - thus, for example, face-recognition, which is handled by a specific, and surprisingly large area in the core of the brain, begins to operate at a very early age. Other intellectual functions slot into place later than this, and some specialists now think that there are probably fairly specific brain functions that come on line at certain periods right through life. What we are interested in is, of course, the language function.
There are reasons for believing that for the basic linguistic capacities such as the construction of grammatical utterances, there is a critical period at about eight or nine years old. The case of Genie, which I have mentioned before, may give us some insight into this phenomenon. She learned to speak after the age of ten. She picked up vocabulary easily, and was able to classify objects, but she never seems to have mastered syntax, and communicated through using single words. Moreover, her language processing seemed to be carried out by her right cerebral hemisphere - neither Broca's nor Wernicke's areas were brought 'on line'.
This is a single case, and we must take great care in building any language acquisition theory on it. However, it does fit in with some observations that suggest that Second Language learners stock their L2 in the right-hand hemisphere, rather than the left - although what is meant by 'stocking' here is rather obscure. However, it should be noted that normal L2 learners, even if they begin so learn the L2 after the age of 10 years old, do much better with the foreign language than Genie did with her first. This leads us to surmise that if it is true that there is a critical age for the entry on-line of the language function, then nevertheless, once it has been activated, it remains available for the acquisition of subsequent languages.
Studies of people who have suffered brain damage lead scientists to believe that the distinction between procedural and declarative memory is physiologically based. Damage to the hippocampus can lead to amnesia - but what is forgotten appears to be declarative rather than procedural. This has lead some observers to suggest that whereas some learning - procedural learning - does occur according to the classical behaviourist model of conditioning, other kinds of learning - declarative learning - occur through more complex cognitive processing. The relevance of this for language learning is not evident, for there is as yet no clear picture of which language skills, if any, are procedural, and which are declarative.
There is also considerable discussion of the nature of intelligence. Thus the American psychologist, Howard Gardner believes that all human beings have several different kinds of intelligence - he identifies linguistic, musical, spatial, logical mathematical, bodily kinaesthetic and personal intelligences - which may not be correlated with each other - thus a person could have a high level of kinaesthetic intelligence, while being low on verbal intelligence, and so on. He cites the cases of 'idiot savants', who, for example, have low general IQ, but are very good at numerical calculation.
Mike Anderson, on the other hand, points to the regularities in cognitive processes. He indicates three such regularities :
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1. The different cognitive abilities do tend to be correlated
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2. The different abilities tend to increase with age at about the same rate.
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3. Individual differences in intelligence remain relatively constant through time - bright babies become bright adults.
He then goes on to suggest that there is a basic processing mechanism that is used by the different kinds of knowledge system. In some people, this mechanism is relatively rapid, whilst in others it is slower. People who process more quickly can use their working memory - which is limited in size - more efficiently, processing more information, and losing less - so they learn more and reason better.
He believes that there are also specific modules which deal with certain kinds of processing necessary to life - vision, syntactical analysis, the ability to distinguish between people and objects - these function in more or less the same way in all human beings.
Finally, there are also specific processors to handle particular learned abilities, but they work in conjunction with the basic processor, so that although we may be slightly better at maths than at word-play, the differences are not great. As we can see, the questions of why we are intelligent, and of how our intelligence works are the subject of discussion and disagreement. The human brain has been called the most complex system in the known universe, and we are far from understanding it fully. What does seem clear is that it is both modular and unified. It has an enormous capacity for learning. We shall turn to the question of how it learns over the next few weeks.
1. A readable account of human evolution will be found in Richard Leakey and Roger Lewin, 'People of the Lake - Man : His Origins, Nature and Future', Penguin, 1981. If you find this question of interest, you should read Chris Knight, 'Blood Relations : Menstruation and the Origins of Culture', Yale University Press, 1991, which will make you very happy. Speculations on the evolution of language are today back in vogue, after having been shunned for almost a century : Terrence Deacon's 'The Symbolic species : The co-evolution of language and the human brain', Allen Lane, 1997, is a stimulating example of the genre - although it makes for tough reading. Jean Aitchison offers an account that is more accessible to the general reader in 'The Seeds of Speech : Language origin and evolution'. Robin Dunbar, 'Grooming, Gossip and the Evolution of Language', faber & faber, 1996, offers a quirky but interesting hypothesis.
2. On neoteny, Stephen J. Gould's essay on the evolution of Mickey Mouse is helpful - see 'A Biological Homage to Mickey Mouse', in 'The Panda's Thumb : More Reflections in Natural History', Penguin, 1980.
3. Harry Jerison, 'The Evolution of the Brain and Intelligence', Academic Press, 1973.
4. An interesting alternative to this is Elaine Morgan's belief that humanity evolved as a semi-aquatic creature. See her 'The Aquatic Ape : A theory of human evolution', Souvenir Press, 1990. By this account, humanity became bi-pedal in order to wade about in rivers and lakes. Monkeys who live on the shores do adopt a standing position more frequently than do their arboreal or savannah dwelling relatives. (But see Jim Moore's critique)